The present invention relates to a method for processing a nano-size element having a carbon network structure as its outer shell and more particularly, to a method for opening the ends of carbon nanotubes.
A carbon nanotube, which is a carbon material having a stereo structure, a nano-size and has carbon network structure as its outer shell, is formed according to well known methods such as the arc discharge method, laser evaporation method or chemical vapor deposition (CVD) technique.
Further, the metallic behavior, electric characteristics such as semiconductor-like behavior and mechanical characteristics including excellent strength combined with light weight, as well as its peculiar structure, have led to a large amount of research directed to carbon nanotubes. Using those characteristics, a wide application is expected in hydrogen, methane absorbing material, selectively absorbing material for use for gas separation based on the pressure swing adsorption method (PSA) or the like, material of fuel cell electrode, material of electrochemical supercapacitor electrode, material of highly functional lithium cell anode, nanodevices and light weight, high-strength composite material. In fact, actual use thereof has already started, for example, admixed with resin, in trays for supporting electronic parts and automobile mounting parts.
Carbon nanotubes include single-walled carbon nanotubes having a single carbon layer structure and multi-walled carbon nanotubes having a two or more carbon layer structure.
The above-mentioned single-walled carbon nanotube is formed with its terminal ends closed by fullerene semi-sphere and, to incorporate some substance into its interior space, these ends need to be removed.
To remove the terminal ends, conventionally, an oxidative acid solution such as nitric acid, or mixed acid is used for reaction with the carbon nanotube. Various chemical reactions, mainly oxidizing reactions, selectively occur at the terminal ends of the carbon nanotube, so that the ends are removed. The principle of this method is considered to be based on the fact that the chemical reactivities of the five-membered ring and six-membered ring composing the carbon nanotube are different in that the five-membered ring has a higher chemical reactivity than the six-membered ring. Alternatively, irradiating with ultrasonic wave so as to increase reactivity (applying ultrasonic vibration selectively destroys the terminal end of the carbon nanotube) or oxidizing at about 400° C. in dry air may be employed.
These methods not only remove the terminal ends, but also oxidize (burn) the nanotube over a wide area, thereby creating the possibility of lower yield or that the peculiar nanotube structure may be destroyed. Thus, although the oxidizing reaction needs to be accurately controlled to remove the ends of carbon nanotubes, the conventional methods are incapable of providing such accurate control.
Further, processes for opening carbon nanotubes individually are not profitable industrially. Therefore, in the present invention, carbon nanotubes are treated as an aggregate and it is intended to remove the ends from each carbon nanotube composing that aggregate. The removal of the ends of carbon nanotubes involves selectively oxidizing the terminal ends because the terminal end of a carbon nanotube has a larger structural distortion than its side portion so that it is more highly reactive in oxidation.
Further, the present invention intends to destroy a specific portion in nano-size substance having a carbon network structure as its outer shell to change the characteristic of the nano-element, thereby enabling new applications.
If the oxidizing conditions are strong (the oxidizing depends on concentration of oxidizing agent, reaction time, heating temperature and the like), the nanotube itself is destroyed, so that best use of the peculiar structure of the nanotube may not be achieved.
In case of heat treatment, for example, uniform oxidation is difficult to achieve, not only for the entire nanotube aggregate, but also at the nano-scale level, because activity of oxygen molecules is essentially at a micro level and because of a temperature gradient in a sample (there is a possibility that a sufficiently oxidized portion and not so oxidized portion may be produced because a temperature differential is created in the sample).